Embryonic stem cells (ESCs) have been widely described in the literature. ESC lines can proliferate indefinitely in an undifferentiated state in vitro. These cells are more accurately described as pluripotent stem cells (PSCs), meaning that they can be stimulated to generate any and all of the cell types present in an organism (e.g., bone cells, muscle cells, brain cells) under suitable differentiation conditions in vitro. PSCs have been isolated from the inner cell mass of the developing murine blastocyst (Evans et al., Nature 292:154-156, 1981; Martin et al., Proc. Natl. Acad. Sci. U.S.A. 78:7634-7636, 1981; Robertson et al., Nature 323:445-448, 1986; Doetschman et al., Nature 330:576-578, 1987; and Thomas et al., Cell 51:503-512, 1987; U.S. Pat. No. 5,670,372). Additionally, non-human primate and human cells with PSC properties have been isolated from the inner cell mass of blastocysts (Thomson et al., Proc Natl Acad Sci USA. 92(17):7844-8, 1995; Thomson et al., Science 282:1145-1147, 1998) and developing germ cells (Shamblott et al., Proc. Natl. Acad. Sci. U.S.A. 95:13726-13731, 1998) (see also U.S. Pat. No. 6,090,622, PCT Publication No. WO 00/70021 and PCT Publication No. WO 00/27995). Human ESC markers have been described, for example, Stage Specific Embryonic Antigen (SSEA)-3, SSEA-4, TRA-1-80 and Octomer-binding transcription factor (OCT)-4, a homeodomain transcription factor.
Transfer of nuclear material from a somatic cell into an enucleated host oocyte is known as somatic cell nuclear transfer (SCNT). SCNT dates back to 1962 when John Gurdon first demonstrated that a differentiated vertebrate somatic cell nucleus (from a larval stage intestinal epithelial cell) could be reprogrammed back into an undifferentiated state after being transferred into an enucleated Xenopus laevis egg and elicit the development of a cloned adult frog (Gurdon, J. Embryol. Exp. Morph. 10: 622-640, 1962).
Nuclear transfer using embryonic blastomeres (i.e., non-somatic cells) as the source of donor nuclei has been successfully used to produce primates (Meng et al., Biol Reprod 57(2): 454-9, 1997; Mitalipov et al., Biol Reprod 66, 1367-73 (2002). However, despite the remarkable progress achieved in the past decade in mammalian SCNT, to date, it has not been possible to obtain non-human primate or human totipotent stem cells (TSCs) that are capable of resulting in a viable fetus or developing into a blastocyst in vitro that subsequently acts as a source of functional PSCs using SCNT. The ability to generate primate, especially human, TSCs and PSCs progeny by SCNT would be useful for many medical purposes, such as autologous transplantation to individuals in need thereof without immunorejection. Accordingly, there remains a long and unfulfilled need to achieve the goal of obtaining pluripotent ESCs by SCNT in primates including, in particular, humans.